We studied the breeding biology of Imperial Cormorant <i>Phalacrocorax atriceps</i> and Rock Shag <i>Phalacrocorax magellanicus</i> at Golfo San Jorge, Argentina, from 1991 to 1993. Dates in which first eggs were observed varied among seasons, being between mid October and mid November for Imperial Cormorants, and between late October and early December for Rock Shags. During 1993, 87% and 86% Imperial Cormorant and Rock Shag pairs, respectively, completed their clutches within four weeks after egg-laying started. The number of eggs per nest recorded one month after the start of egg laying for both species was statistically different among years. The average number of eggs per nest was similar between Imperial Cormorants and Rock Shags during 1993 (2.4 vs. 2.1, respectively) but significantly different during 1991 (1.9 vs. 0.8). During two of the study seasons the volume of the third egg was significantly smaller than the first and second eggs. The average length of the incubation period for Imperial Cormorants was estimated at 27.9 days. Hatching success of Imperial Cormorants was 38.2% (1991) and 52.5% (1992). Chick weight increase and culmen growth showed maximum values during the second and third weeks after hatching, reaching asymptotic values at 60 days. Chick tarsus growth reached maximum values during the first week and asymptotic values at 30 days. During 1993 mean number of chicks fledged per nest was 1.13 for Imperial Cormorants and 0.83 for Rock Shags.

Diets of breeding Imperial Cormorants Phalacrocorax atriceps were studied at two breeding colonies, Islas Blancas and Isla Arce, located approximately 30 km apart in an area subject to increasing fishing pressure off Central Patagonia, Argentina. The goal was to assess differences between locations and diet variation among stages of the breeding cycle. Pellet casts (403 and 358 pellets per colony, respectively) were collected from November 2002 to February 2003. Analyses of the pellets revealed that Imperial Cormorants at Islas Blancas and Isla Arce fed on at least 25 and 23 prey types, respectively. Fish showed the highest frequency of occurrence at both colonies (> 70%), followed by crustaceans and molluscs. Of the fish prey, Merluccius hubbsi (2271%), Engranlis anchoita (1651%) and Raneya brasiliensis (548%) showed the highest frequencies of occurrence, depending on the colony and breeding stage. At Islas Blancas, the consumption of fish and crustaceans was similar among breeding stages (incubation, young chicks and old chicks), while it was significantly different at Isla Arce. Overall contribution by frequency of occurrence showed that M. hubbsi was the most frequent prey at Islas Blancas (58%), and E. anchoita and Pleoticus muelleri were more frequent at Isla Arce (48 and 45%, respectively). Also, overall contribution by mass of the main fish prey indicated differences between colonies. Given the commercial value of the main prey species, cormorant feeding requirements and spatial ecological needs should be included as considerations in coastal fisheries management and future development.

Energy management during the breeding season is crucial for central place foragers since parents need to feed themselves and their offspring while being spatially and temporally constrained. In this work, we used overall dynamic body acceleration as a measure of activity and also to allude to the foraging energy expenditure of breeding Imperial cormorants Phalacrocorax atriceps. We also analyzed how changes in the time or energy allocated to different activities affected the foraging trip energy expenditure and estimated the daily food requirements of the species. Birds spent 42 % of the total energy flying to and from the feeding areas and 16 % floating at sea. The level of activity underwater was almost 1.5 times higher for females than for males. The most expensive diving phase in terms of rate of energy expenditure was descending though the water column. The total foraging trip energy expenditure was particularly sensitive to variation in the amount of time spent flying. During the breeding season, adult cormorants breeding along the Patagonian coast would consume approximately 10,000 tons of food.

Several studies have applied induced regurgitations to characterize the diet of cormorants, but none have presented quantitative information indicating complete stomach contents were obtained. Our goal was to test the value of induced regurgitations for the assessment and monitoring of Imperial Cormorant (Phalacrocorax atriceps) diet. Stomach samples were obtained from male and female breeding adults bringing food back to the colony during the chick rearing stage (n = 22) at Isla Arce, Argentina. Samples were obtained through induced regurgitation, and immediately afterward each individual was flushed with sea water. The diet of the Imperial Cormorant consisted of at least 23 prey taxa, mostly fish complemented by crustaceans, cephalopods and polychaetes. However, only Argentine anchovy (Engraulis anchoita) and rockcods (Patagonotothen spp.) showed a significant contribution by mass (70.7% and 25.3%, respectively). Analysis of similarity indicated that prey composition between samples obtained by induced regurgitation and those obtained by combining regurgitation followed by stomach flushing were similar in both the numerical frequency of all prey taxa recorded and the contribution by mass of the main prey. Our results show that induced regurgitation provides complete stomach contents, and thus validates the use of this technique for quantifying Imperial Cormorant diet composition.

Several studies have applied induced regurgitations to characterize the diet of cormorants, but none have presented quantitative information indicating complete stomach contents were obtained. Our goal was to test the value of induced regurgitations for the assessment and monitoring of Imperial Cormorant (Phalacrocorax atriceps) diet. Stomach samples were obtained from male and female breeding adults bringing food back to the colony during the chick rearing stage (n = 22) at Isla Arce, Argentina. Samples were obtained through induced regurgitation, and immediately afterward each individual was flushed with sea water. The diet of the Imperial Cormorant consisted of at least 23 prey taxa, mostly fish complemented by crustaceans, cephalopods and polychaetes. However, only Argentine anchovy (Engraulis anchoita) and rockcods (Patagonotothen spp.) showed a significant contribution by mass (70.7% and 25.3%, respectively). Analysis of similarity indicated that prey composition between samples obtained by induced regurgitation and those obtained by combining regurgitation followed by stomach flushing were similar in both the numerical frequency of all prey taxa recorded and the contribution by mass of the main prey. Our results show that induced regurgitation provides complete stomach contents, and thus validates the use of this technique for quantifying Imperial Cormorant diet composition

Intersexual differences in the foraging behaviour have been examined in several seabird species, especially those exhibiting sexual size dimorphism. We studied intersex behavioural differences in the Imperial Cormorant (Phalacrocorax atriceps), a size dimorphic seabird. Twenty adults (11 females and 9 males), breeding at Punta León (43°04′S; 64°2′W), Chubut, Argentina, were equipped with loggers to measure tri-axial acceleration and depth, to calculate the foraging trip time/activity budgets and diving behaviour. Both sexes had similar foraging trip durations, spending a similar proportion of the foraging time flying and floating on the sea surface. However, females spent more time underwater, executing more and shallower dives. Females also recovered more quickly than males from dives performed to depths of less than 30 m and spent more time foraging along the bottom at any depth than males. We conclude that if allometric effects affect the foraging behaviour of Imperial Cormorants, they only do so during diving because no differences were observed in the total amount of time sexes spent flying or foraging.

Plasma biochemistries provide a complementary method for assessing physiological and nutritional status of free-ranging wild birds. Triglycerides, total protein and alkaline phosphatase were determined in 110 free-living Imperial Cormorant (Phalacrocorax atriceps) chicks aged 16-35 days, at Punta León (Argentina) during 2010 and 2011. Body mass at 30 days of age ("pre-fledging body condition", 2010 only) and body mass corrected by tarsus length at the time of blood sampling ("current body condition", 2011 only) were also determined. Variability of parameters by sex, hatching order, survival, age and breeding season was assessed, and the relationship between biochemical and morphometric indices was also explored. Morphometric indices were higher in A-chicks (pre-fledging body condition also varied with sex), and explained 35-55% of B-chick survival. Biochemistries differed significantly between breeding seasons, being higher in 2011. Alkaline phosphatase increased with age, and total protein was higher in A-chicks. Triglycerides and total protein accounted for 26% and 30%, respectively, of variation in current body condition; however, they did not forecast pre-fledging body condition. Lastly, total protein levels predicted B-chick survival (higher levels in surviving B-chicks), but their prognostic value was relatively low. The results suggest that unlike morphometric indices, the biochemistries chosen are valuable to assess individual body condition at the time of sampling, yet their applicability for predicting chick survival requires further evaluation.

Behavior by a foraging seabird during the breeding season can be examined by analyzing time invested throughout the foraging route to determine the presence of Area-Restricted Search (ARS) as well as other characteristics related to the shape of the foraging path and activity (flying, resting or diving). Forty-six Imperial Cormorants (Phalacrocorax atriceps), 18 males and 28 females breeding at Punta León, Argentina (43°04´S; 64°2´W), were fitted with GPS loggers recording one foraging trip (sampling interval: 1 second) in 2004, 2005, 2007 and 2008. Trip duration was longer, on average, for females (6.3 ± 1.9 h) than males (5.3 ± 1.6 h) (Mann-Whitney U test z 1,45 = 2.23, P = 0.026), but year had no significant effect on any of the path characteristics. ARS was detected in forty-three individuals, twenty-two of which made smaller scaled searches nested within larger ARS areas. Search scale was not correlated to maximum distance reached and did not differ between sexes nor years. Cluster analysis separated four types of behavior: short direct return trips (N = 7), long direct return trips (N = 31), random flight searches (N = 6) and loops (N = 2), with each behavioral type present in both sexes. Behavioral variability within a population may be due to differences in targeted prey type and spatio-temporal stability during the season, as well as to individual physiological constraints and life-history traits linked to how individuals search for food at sea.

We conducted a serologic survey for selected infectious agents on two sympatric cormorants, the Imperial Cormorant (Phalacrocorax atriceps) and the Rock Shag (Phalacrocorax magellanicus). Blood was collected from 267 Imperial Cormorants and 106 Rock Shags at 17 colonies along the Patagonia Atlantic shore during nine breeding seasons (1994, 1999?2001, 2005? 2008, 2010). Antibodies to four pathogens were common to both species and frequently observed: avian paramyxovirus type 1 (56.2% of Imperial Cormorants and 55.8% of Rock Shags); avian adenovirus (67.3% of Imperial Cormorants and 40.2% of Rock Shags); infectious bronchitis virus serotypes IBV-41, IBV-46, IBV-99, and IBV-JMK (52.5% of Imperial Cormorants and 63.9% of Rock Shags); and Salmonella pullorum (17.7% of Imperial Cormorants and 6.6% of Rock Shags). Antibody prevalence for these pathogens varied significantly between species, except for avian paramyxovirus type 1. Exposure to avian paramyxovirus type 1 and all serotypes of infectious bronchitis virus varied significantly among seasons in both species. In contrast, the sporadic occurrence of positive titers suggest that cormorants had occasional exposure to Aspergillus spp. (3.3% of Rock Shags, only in 2000), avian paramyxovirus type 3 (4.8% of Rock Shags, only in 2008), Chlamydophila spp. (0.8% of Imperial Cormorants, only in 2010), and avian reovirus (1.1% of Rock Shags, only in 1999; 29.4% of Imperial Cormorants, in 2008 and 2010). Both species were antibody negative for avian encephalomyelitis virus, avian influenza virus, avian laryngotracheitis virus, avian paramyxovirus type 2, and infectious bursal disease virus. We provide the first information on pathogen exposure, indicated by detection of antibody in blood samples, for two sympatric species of South Atlantic cormorants. To determine major causes of morbidity and mortality in these birds future efforts should focus on necropsy surveys in cormorant colonies.

Cormorants and shags (Phalacrocoracidae) are sexually monomorphic in plumage but dimorphic in size with males larger and heavier than females. Such size dimorphism has been capitalized upon for several species in the family to sex adults by using discriminant analysis applied on the morphometric measurements. Despite that, few studies have analyzed the development of sexual size dimorphism during chick growth. In this study, sexspecific growth was assessed in chicks of the Imperial Cormorant (Phalacrocorax atriceps) by analyzing the development of body mass, bill length, head length, tarsus length and wing length measured on 80 chicks sexed by DNA-based technics. Fieldwork was performed during the 2004 breeding season at Punta León, Patagonia, Argentina. In addition, discriminant analyses were performed to obtain functions to determine the sex of fledglings. Males had higher asymptotic values and growth rates than females for all measurements considered, even though the arising of dimorphism varied among morphometric characteristics (10-40 days). Discriminant functions to determine the sex of chicks at 30, 35 and 40 days of age were obtained. All functions included tarsus length and head length as variables, correctly classifying 88-94% of chicks. Our findings show the Imperial Cormorant to be an interesting model for evaluating the potential consequences of sexual size dimorphism on chick survival and fledging condition depending on brood sex composition.